在sort函数中，我们根本没有触及subMenu，也不清楚为什么要拆分prop，以及长度与任何内容有什么关系，但是基于示例输入和预期输出，它可以简单得多：

// sort an array of objects based on the value of those objects' `prop`
// creates a new array with [...] because `sort` mutates. this could be changed
// to use partial application if you'll have the same logic for other properties
// in the future.
const sortByProp = (prop, xs) =>
  [...xs.sort((a, b) => a[prop] - b[prop])]

// sort the parent array based on some prop, and subMenu's array based on
// the same prop. if `subMenu` can possibly change down the line, that should be
// moved to an argument as well.
const sort = (prop, xs) =>
  sortByProp(prop, xs.map((x) => ({
    ...x,
    subMenu: sortByProp(prop, x.subMenu)
  })))

// test
console.log(
  JSON.stringify(
    sort('order', testInput), null, 2
  )
)

根据order属性按升序对数组的内部对象进行排序。

试试这个

var MainObj = [{
    "title": "Merchant",
    "order": 1,
    "subMenu": [{
            "subMenu1": "Initiate2",
            "order": 2
        },
        {
            "subMenu2": "Initiate1",
            "order": 1
        }
    ]
}]

var sort = function(prop, arr) {
    prop = prop.split('.');
    var len = prop.length;

    arr.sort(function(a, b) {
        var i = 0;
        while (i < len) {
            a = a[prop[i]];
            b = b[prop[i]];
            i++;
        }
        if (a < b) {
            return -1;
        } else if (a > b) {
            return 1;
        } else {
            return 0;
        }
    });

    arr.forEach(function(obj) {
        obj.subMenu.sort(function(a, b) {
            var i = 0;
            while (i < len) {
                a = a[prop[i]];
                b = b[prop[i]];
                i++;
            }
            if (a < b) {
                return -1;
            } else if (a > b) {
                return 1;
            } else {
                return 0;
            }
        });
    });

    return arr;
};

console.log(sort('order', MainObj));

输出：-

[
  {
    "title": "Merchant",
    "order": 1,
    "subMenu": [
      {
        "subMenu1": "Initiate1",
        "order": 1
      },
      {
        "subMenu1": "Initiate2",
        "order": 2
      },
      
    ]
  }
]